The present invention relates to the field of electric energy storage systems comprising an inverter. These energy storage systems comprise electric energy storage units and are often coupled to an AC network to store electric energy when the AC network has too much or does not need any and to restore the electric energy to the AC network when it needs it. This excess electric energy, which is temporarily stored in the electric energy storage system to then be provided to the AC network, may either come from the AC network or from energy sources, which are preferably renewable, for example such as solar panels or wind turbines.
In an electric energy storage system of a certain size, which must be connected to an AC network, several electric energy storage units are used. These electric energy storage units may be used individually, in a group or in their entirety, at a given moment, depending on the needs of the AC network. At least one inverter, which is a DC/AC converter, is positioned between the electric energy storage units on the one hand and the AC network on the other hand.
The problem then arises of the number of inverters to be used and how to connect the inverter(s) to the electric energy storage units. This is particularly more pronounced inasmuch as the voltages of the electric energy storage units are not necessarily balanced relative to one another, for several reasons. First, the electric energy storage units of a same electric energy storage system may use different technologies or may have different behaviors, for example a power battery behavior and an energy battery behavior. Additionally, the voltage level of certain electric energy storage units may be incompatible with the voltage level of the AC network and may require additional adapter components. For example, this may involve the need to add a boost transformer to ensure the operation of a battery with a low output voltage with an AC network with a high network voltage, for example 600 VDC and 480 VAC.
According to a first prior art, for example described in FIG. 1, which will be outlined later, it is known to connect, in parallel on an AC voltage network, several associations each comprising a battery directly serially connected with an inverter, each inverter being sized for the associated battery. One drawback of this first prior art is the complexity and overall cost of the electric energy storage system, as it comprises several inverters, the inverter being a relatively complex and expensive component.
According to a second prior art, for example described in Chinese patent application CN 102122826, it is known to connect, in parallel at the input of a shared inverter, several associations each comprising a battery in series with a chopper, which is a DC/DC converter. Although using a shared inverter, oversized relative to each of the choppers that are connected in parallel to its input, makes it possible to save in terms of cost and complexity, the addition of several choppers nevertheless creates losses in terms of cost and increased complexity. The cost and complexity balance is positive, but only moderately so. Further, the use of complex choppers comprising many power switches reduces the effectiveness of the power balance.
According to a third prior art, for example described in patent application US 2011/0133556, it is known to connect, in parallel to the input of a shared inverter, several associations each comprising a battery in series with a chopper, which is a DC/DC converter. Although using a shared inverter, oversized relative to each of the choppers that are connected in parallel to its input, makes it possible to save in terms of cost and complexity, the addition of several choppers nevertheless creates losses in terms of cost and increased complexity. The cost and complexity balance is positive, but only moderately so. Additionally, this facility relates to an extra energy storage system for powering, by the AC network, of an installation of modest size, for example an individual home. It may not be used to power the AC network itself.